An established procedure for making all-aromatic polyesters of fiber-forming molecular weight is thermally reacting (melt polymerization) an aromatic diacetate with an aromatic dicarboxylic acid. This reaction liberates acetic acid which is corrosive and requires the use of special corrosion resistant equipment. A further disadvantage is that esters of dihydric phenols or of aromatic diacids are more costly than the unesterified reactants. A polyesterification wherein the aromatic diacid and dihydric phenol are directly reacted with each other is of advantage since by-product water presents far less a disposal problem than phenols or acetic acid involved with ester reactants.
While preparation of polyethylene terephthalate and of monomeric diesters by catalyzed thermal esterification is known, there are frequent reports in the prior art, e.g., Polymer, V.15 August, 1974, page 530 and Kantor et al. U.S. Pat. No. 3,160,602 that direct polycondensation of a dihydric phenol with an aromatic dicarboxylic acid is impracticable or does not work. By "direct" polymerization is meant that the unesterified aromatic dicarboxylic acid and dihydric phenol are reacted to form the polymer. This is to be distinguished from the use of derivatives of such reactants e.g., diesters thereof, or the use of materials which form such derivatives in situ, e.g., diphenyl carbonate and aromatic dicarboxylic acid. As mentioned above, use of the derivatives results in formation of undesirable by-products.
U.S. Pat. No. 3,972,852 discloses a catalytic process for manufacture of polyesters from an aromatic dicarboxylic acid, or ester thereof, a dihydroxyaromatic compound and certain glycol compounds. The catalyst employed was titanium tetrabutoxide. While the patent states that aromatic acid or an ester-forming derivative may be used, either the diphenyl ester was used or the aromatic acid in combination with diphenyl carbonate was used. The combination of the aromatic acid and diphenyl carbonate yields the diphenyl ester.
A process for increasing the rate of direct polymerization, thereby resulting in polymer of increased molecular weight in less time and without the formation of corrosive or other undesired by-products, is a worthwhile objective.